Heat and Winter Solstice Linked to Fatigue and REM Sleep Disruption

Rising ambient temperatures in summer force the body to activate thermoregulatory mechanisms such as peripheral vasodilation and increased heart output. Dehydration and the extra cardiovascular load reduce exercise tolerance and strain the brain and muscles.

Hot nighttime temperatures also increase micro‑awakenings, sharply cutting the restorative REM phase of sleep. The loss of REM sleep leaves people feeling weak, sluggish and mentally foggy the next day.

The winter solstice on December 21 shortens daylight in the Northern Hemisphere, triggering an early rise in melatonin production. Higher melatonin levels can lower mood and disturb normal sleep patterns, adding another layer of fatigue during the longest night of the year.